This invention relates to a Doppler radar designed to operate over a wide bandwidth.
For many years it has been considered desirable for military radars to be able to vary their nominal radar carrier frequency of transmission as widely and as agilely as posible in order to elude jamming and other hostile attentions. Such radars have hitherto generally been restricted to an agile range, or bandpass, of less than one octave, and are able generally to transmit on only one radar carrier frequency at once. These restrictions apply because known efficient means of amplification and control of radar frequency power tend to produce harmonic distortions and intermodulation products (especially if more than one radar carrier frequency is amplified simultaneously), detrimental to the radar and to other band users. Conventional radars employ waveguide components with sufficiently restricted bandpass characteristics so that the unwanted harmonic distortions and intermodulation products are not transmitted to any serious degree. Wideband systems, on the other hand, will transmit many of the unwanted signals. Only those radars which are provided with two separate antennas have hitherto been generally able to transmit and receive simultaneously.
Modern radar systems are required to do more than detect the presence of reflecting objects (targets) in the path of the radar beam as directed by the antenna. Modern radars are additionally required to determine quantitative information, e.g., about the nature, position and velocity of each target detected. The required information may be computed by quantitative comparison of the radar echo signals reflected from targets with the form of the radar transmission signals. This comparison and the computation of the required information is generally termed “Doppler processing.”
It is well known that Doppler processing yields target information which is qualified by a degree of uncertainty and ambiguity, in that each computation of a target attribute yields, in general, more than one value. It is also possible to ‘lose’ actual targets or ‘find’ non-existant ones. For these reasons it is generally necessary to confirm all measurements computed by Doppler processing by taking a second ‘look’ in each beam direction, using different radar transmission signals so that the Doppler processing computations are sufficiently different to make it unlikely that any identical ambiguities and errors result from both looks. Where there are initially many ambiguities, a third or even a fourth look may be required before there can be high confidence that all targets present in the radar beam have been detected and that all relevant information has been accurately and unambiguously computed. Since a conventional radar can use only one radar transmission at a time, multiple looks must take place sequentially so additional time must be spent searching each beam direction. A conventional Doppler radar therefore generally requires more time, to search a given area of sky, than a simple non-Doppler radar of otherwise comparable characteristics.